The National Academy of Sciences has recognized the need for minerals in human nutrition, and has classified them into two groups: those needed in the diet at levels of 100 mg/day or more, and those needed in amounts no higher than a few mg/day. The latter are called trace elements. Either group may contain elements not present in adequate quantities in the diet. The development of exceedingly sensitive methods of analysis has greatly facilitated the advances in trace element research in recent years. Important new advances are being made in our understanding of the role of mineral elements in the structure of protein and in the activity of many enzymes. Certain criteria have been established to determine the essentiality of a mineral element for human requirements, and some of the factors considered are as follows:
1. The mineral must be more or less ubiquitous, available to and absorbed by animals and plants.
2. The chemical nature must be compatible with some physiological function.
3. The atomic number should fall within those known to be essential to health.
4. It must cross the placental and mammary barriers.
5. It should be of low toxicity when administered in the form in which it occurs naturally.
6. A homeostatic controlling mechanism should be present.
There is reason to believe that the social and physical changes in a man's environment occurring as a consequence of technological developments could have important effects upon trace elements intake by man. The nature and magnitude of such changes are not completely known, and their potential importance is insufficiently appreciated. The World Health Organization has recommended that: (1) an international collaborative program be designed to obtain reliable information on the trace element content of foods; (2) systems be established to monitor the changes in relation to agricultural and industrial development; and (3) full recognition be given to the needs for trace elements in developing standards for foods.
Two trends may lead to an imbalance of trace element nutrition in the future. First, increasing consumption of highly refined or processed foods substantially reduces the intake of essential micronutrients, unless those foods are fortified to concentrations at least equal to those naturally occurring in the foods they replace. Second, human exposure to heavy metal pollutants for which no known need has been established may alter requirements for these trace elements. These essential biological metals are the inorganic counterparts of the essential biological organic nutrients, the vitamins. Unlike vitamins, they cannot be snythesized by living organisms and must be present in the diet.
The four elements that are needed in quantities of 100 mg/day or more, are sodium, potassium, calcium, and magnesium. Sodium is the principal cation of extracellular fluid. The usual adult intake of sodium is 100-300 meq/day, equivalent to 6-18 gm of sodium chloride. Both the body content and body fluid concentration of sodium are homeostatically controlled, moderate intakes being rapidly excreted in the urine while a reduction in intake causes excretion to drop to a very low level.
Potassium is the principal intracellular cation as well as being present in lesser concentrations in the extracellular fluids, and is involved primarily with cellular enzyme function. A deficiency of this cation retards growth. As with sodium, efficient homeostatic mechanisms allow for a fairly wide range of intakes. Adults require about 2.5 g/day. An adequate potassium intake is important during prolonged intravenous feeding, in cases of severe diarrhea, in diabetic acidosis, and in therapy with certain drugs.
An adult body contains about 1200 g of calcium, almost all of which is in the skeleton. Despite the seeming permanence of the mineral deposits in bone, the calcium is constantly being formed and resorbed. About 700 mg of calcium enter and leave the bones each day. The amount of calcium outside bone does not exceed 10 g in the adult. Nevertheless, this small amount of calcium has an important role in controlling the excitability of peripheral nerves and muscle. Calcium is necessary for blood coagulation, for myocardial function, muscle contractility, and for the integrity of various membranes. The importance of calcium in these functions is reflected in the precision with which plasma calcium is regulated. The fluctuations have been reported to be + or - 3%! The level of protein intake influences the absorption of calcium, but in general, excess calcium in the diet may not be absorbed. The FAO/WHO recommended a daily allowance of 400-500 mg/day. Calcium supplementation has induced calcium retention and relieved symptoms of osteoporosis. Of course, pregnancy and lactation increase the requirements for this element.
Magnesium is selectively concentrated in living cells and is also stored in bone. Next to potassium, it is the predominant cation in the cell, and is an essential part of many enzyme systems. It is important in maintaining function in nerves and muscles. With advances in methodology for magnesium analysis, the importance of this element in various disorders has been recognized. Low levels of magnesium occur in alcoholism, and hypomagnesemia has been implicated as the cause of tremors and convulsions in children. There is no evidence that larger than normal intakes are harmful. Magnesium and calcium share several control functions, and magnesium excretion falls as dietary calcium is reduced. Magnesium lack leads to neuro-muscular dysfunction and is sometimes accompanied by behavioral disturbances. The generally recommended allowance is about 300-350 mg/day.
At the present time, 14 trace elements are believed to be essential for human nutrition. These are iron, iodine, copper, zinc, manganese, cobalt, molybdenum, selenium, chromium, nickel, tin, silicon, fluorine, and vanadium. It is probable that other elements will be added to this list as experimental techniques are further refined and applied. Five of the elements in this list have emerged as being essential only in the past 6-7 years. Also, within 5 years of the demonstration of the essential function of chromium and its relation to insulin metabolism, marginal deficiency of this element affecting the health and well being of certain population groups in the U.S.A. was reported by several groups of workers. Further, a supposed adequate level of an essential trace element will be affected by the extent to which it interacts with other trace elements in the body, and with other elements in the environment. The importance of balance in the ratios of trace elements provided can be demonstrated by the following examples of biological interactions: Copper is essential for some aspects of iron utilization. Molybdenum increases absorption of fluoride. Selenium compounds have been shown to counteract the toxicity of organic and inorganic compounds of mercury. An inverse statistical association has been shown to exist between the hardness of drinking water and cardiovascular disease mortality, and it is not now possible to relate this to one specific element. The elements considered most likely to show a relationship are magnesium, calcium, chromium, copper, zinc, selenium and cadmium. Low dietary intakes of calcium and iron may increase the absorption and therefore the toxicity of lead. Zinc has been shown to counteract some of the adverse effects of cadmium, and can reverse cadmium induced hypertension in animals. A low or marginal copper intake may also decrease the tolerance to cadmium. Selenium has a protective effect against the toxicity of cadmium. Magnesium depletion has been shown to impair the homeostasis of both potassium and calcium. Interactions between cobalt and iodine have been demonstrated, and the cobalt concentration in soils, food, and water is inversely related to the incidence of goiter in man and farm animals.
Varied developments in agricultural technology can affect the trace element content of foods. The introduction of new varieties of plants, the use of agricultural chemicals, alterations in feeding practices for farm animals, and other new techniques designed to increase food production and yields may result in changes in the composition of basic items of food. Useful technological innovations are providing new types of food ingredients from unconventional sources. This inevitably results in changes in the dietary supply of trace nutrients. Similarly, with affluence and industrialization, a reduction in calories and energy expenditure takes place, and this reduction may reduce the supply of necessary nutrients. Similarly, there is an increase in the consumption of processed and refined foodstuffs, extracts, and formulated products. Such actions may reduce the content of certain trace elements, increase that of others, or affect their biologic availability. Magnesium, calcium, chromium, copper, zinc and selenium are considered to be the trace elements most likely to show relationships to the mortality, morbidity, and geographical distribution of cardiovascular diseases.
The following comments will serve to briefly summarize the present state of knowledge with respect to the aforesaid essential trace elements.